The present invention is in a general way concerned with mobile radiocommunication systems.
The present invention is more particularly concerned with power control techniques used in such systems to improve performances (in terms of quality of service, of capacity, . . . etc.) despite the movements of users, i.e. despite continuous changes in their respective locations with respect to fixed infrastructures in such systems.
The present invention is in particular applicable to mobile radiocommunication systems of CDMA (xe2x80x9cCode Division Multiple Accessxe2x80x9d) type. CDMA is a multiple access technique which makes it possible for several users to be simultaneously active on an same frequency, using different spreading codes.
As is known, CDMA systems use two types of power control techniques, a so-called open-loop power control technique, and a so-called closed loop power control called open-loop power control technique, and a so-called loop power control technique (also called hereinafter CLPC). These power control techniques may be recalled for example for the uplink transmission direction, i.e. from MS (xe2x80x9cMobile stationxe2x80x9d) to BTS (xe2x80x9cBase Transceiver Stationxe2x80x9d). In the open-loop power control, a MS transmit power is controlled based on the power received by this MS from a BTS. In the CLPC, a MS transmit power is controlled based on the transmission quality of the link between this MS and a BTS, as estimated at this BTS.
The transmission quality of a link between a MS and a BTS depends on the ratio of the received signal power and the interference power, also called SIR (Signal-to-Interference Ratio). When the SIR of a MS is low, or equivalently when the powers of the other MSs are much higher than its power, its performances dramatically decrease. The CLPC algorithm enables to keep the SIR of each user as constant as possible.
The principle of the CLPC algorithm is that the BTS periodically estimates the SIR of the received signal from each MS, and compares this estimated SIR to a target SIR (SIRtarget). If the estimated SIR is lower than the target SIR, the BTS sends a command to the MS for the MS to increase its transmit power. Otherwise, the BTS sends a command to the MS for the MS to decrease its transmit power. The target SIR is chosen by the BTS as a function of the required quality of service.
To be efficient and track the variations of the SIR as closely as possible, in particular in fast changing environments, the CLPC needs to be rapid; as an example, in third generation systems like for example UMTS (xe2x80x9cUniversal Mobile Telecommunications Systemxe2x80x9d), power control commands are typically sent to a MS every slot in a frame (a slot being an elementary time unit in a packet data unit, or frame, transmitted in such a system, the frame duration being typically equal to 10 ms, and the slot duration to {fraction (1/16)} of the frame duration).
Now there are some situations in mobile radiocommunication systems, in which transmission has to be momentarily interrupted.
As an example, in CDMA systems, downlink transmission from a BTS to a MS may momentarily be interrupted to allow this MS to make measurements on frequencies other than the one used for this downlink transmission (in particular for handover preparation purposes, in particular inter-frequency handovers). Such a transmission mode including transmission interruptions is also referred to as xe2x80x9cslotted modexe2x80x9d in third generation systems like UMTS for example. A transmission interruption may last several slots in a frame. During these transmission interruptions, the CLPC is interrupted. Thus, the BTS does not send any more power control commands to the MS, and the uplink signals from this MS are no longer power controlled. Uplink transmission may simultaneously be interrupted, but in either case, as a result, the efficiency of the CLPC is significantly decreased, and the performances of the system may be seriously degraded.
Therefore there is a need to avoid as far as possible such degradations of performances, due to such transmission interruptions.
It is noted that reference WO 9836508 discloses a power control method for DTX, or discontinuous transmission, wherein power control commands are changed according to a DTX state, in particular the frequency of power control commands being lowered in a DTX state. Besides, it is mentioned in this reference that errors caused by slower power control may be compensated for by increasing the size of power control steps, to be bigger than the size of quick power control steps. However in this reference, power control is not interrupted during the DTX state. It is recalled that DTX, or discontinuous transmission, is a transmission mode according to which, in order to reduce interferences inside the system, transmission of radio signals is stopped when there is no information to be transmitted, for example when there is no voice activity.
An object of the present invention is a method for improving performances of a mobile radiocommunication system using a power control algorithm, said method essentially comprising:
detecting that transmission is resumed after a transmission interruption,
upon such a detection, implementing said power control algorithm with at least one modified parameter, for a given duration, said at least one modified parameter and said given duration being determined so as to compensate for the effects of said transmission interruption on power control.
According to another object of this invention, said at least one parameter is the power control step size of the algorithm.
According to another object of this invention, said at least one modified parameter is an increased power control step size.
According to another object of this invention, said at least one modified parameter and said given duration have predetermined values.
According to another object of this invention, said given duration is determined to be passed when a given condition is fulfilled.
According to another object of this invention, said given condition is determined to be fulfilled based on successive power control results with said at least one modified parameter.
According to another object of this invention, said given condition is that two consecutive power control commands obtained with said at least one modified parameter are opposed.
According to another object of this invention, said at least one modified parameter and said given duration are determined based on statistics on power control results for a period before said transmission interruption.
According to another object of this invention, said power control is performed in the uplink transmission direction of said mobile radiocommunication system.
According to another object of this invention, said power control is performed in the downlink transmission direction of said mobile radiocommunication system.
According to another object of this invention, said mobile radiocommunication system is of CDMA type.
Another object of this invention is a mobile radiocommunication network entity (such as in particular BTS) for performing such a method.
Another object of the present invention is a mobile station (MS) for performing such a method.
According to another object of the invention, a mobile radiocommunication network entity comprises, for performing said method in said uplink transmission direction:
means for performing a power control algorithm modified according to said method,
means for sending corresponding power control commands to a mobile station.
According to another object of the invention, a mobile station comprises, for performing said method in said uplink transmission direction:
means for receiving power control commands from a mobile radiocommunication network entity, according to said method.
According to another object of this invention, a mobile radiocommunication network entity comprises, for performing said method in said uplink transmission direction:
means for implementing a power control algorithm, not modified according to said method,
means for sending corresponding power control commands to a mobile station.
According to another object of this invention, a mobile station comprises, for performing said method in said uplink transmission direction:
means for receiving power control commands from a mobile radiocommunication network entity, and for modifying said power control commands according to said method.
According to another object of this invention, a mobile station comprises, for performing said method in said downlink transmission direction:
means for performing a power control algorithm, modified according to said method,
means for sending corresponding power control commands to a mobile radiocommunication network entity.
According to another object of this invention, a mobile radiocommunication network entity, comprises, for performing said method in said downlink transmission direction:
means for receiving power control commands from a mobile station according to said method.
According to another object of this invention, a mobile station comprises, for performing said method in said downlink transmission direction:
means for implementing a power control algorithm, not modified according to said method,
means for sending corresponding power control commands to a mobile radiocommunication network entity.
According to another object of this invention, a mobile radiocommunication network entity comprises, for performing said method in said downlink transmission direction:
means for receiving power control commands from a mobile station, and for modifying said power control commands according to said method.